If it is that? a diaphragm only costs a few quid then about 60 odd quid for the engineer to fit it. The shell of the diverter valve (the brass part) should be fine as the boiler is only 3 years old. They could turn up and it be somethimg else, but diaphragm sounds like most likely cause. By the way the pump does come on when heating is turned on doesnt it? does not make any strange noises?